1. Field of the Invention
The present invention relates to an apparatus and method for estimating a position of a mobile communication terminal and, more particularly, to improving a plotting function by combining position estimating methods.
2. Background of the Related Art
There are various methods for estimating a position of a terminal in a mobile communication system. Among the methods applied to a Code Division Multiple Access (CDMA) cellular system are a method that modifies a terminal and a method that does not modify a terminal.
Representative of the method that modifies the terminal is a plotting method, based on a terminal. This method requires a Global Positioning System (GPS) receiver to be installed in a mobile communication terminal carried by a user.
FIG. 1 is a view showing a plotting method for a mobile communication terminal using a GPS satellite. When the GPS receiver 101 installed in the mobile communication terminal is operated, by pushing a predetermined button on the mobile communication terminal, it receives a signal from the GPS satellites 102. Based on these signals, GPS receiver 101 calculates the distances (r11, r12, r13, and r14 ) between the mobile communication terminal and the respective satellites. To estimate a three dimensional position, GPS receiver 101 should receive signals from more than four GPS satellites, and to estimate a position in two dimensions, GPS receiver 101 should receive signals from more than three GPS satellites. A mobile communication exchanger 103 calculates the position of the mobile communication terminal using additional information received from the GPS satellite 102, and after that, the calculated position is automatically transmitted to an application server 104 using the CDMA cellular network.
On the other hand, there is a terminal position estimating method based on a network that does not need to modify the terminal. The above method can be divided into a Time of Arrival (TOA) method, using the arrival time of a signal transmitted to a respective base station; a Time Difference of Arrival (TDOA) method, using arrival time differences of mobile communication terminal signals transmitted to respective base stations; an Angle of Arrival (AOA) method using arrival angles of signals transmitted to the respective base stations; and a Radio Frequency Finger Printing (RFFP) method, using intensity data of a base station signal, which is measured within divisions of the service area in advance.
FIG. 2 is a view showing the TOA method that uses the arrival time of the signal. A plurality of base stations 202, around a mobile communication terminal 201, receive a signal transmitted from the mobile communication terminal 201. Then, the respective base stations 202 measure the arrival time of the transmitted signal to calculate distances (r21, r22, and r23) between mobile communication terminal 201 and base stations 202. Virtual circles, centered around the respective base stations 202, are drawn having respective radii of r21, r22, and r23 such that the position where the circles cross each other is the estimated position of mobile communication terminal 201.
FIG. 3 is a view showing the AOA method, using an arrival angle of the signal. A plurality of base stations 302, around a mobile communication terminal 301, receive a signal transmitted from mobile communication terminal 301. The respective base stations 302 measure the arrival angle (Θ1, Θ2) of the transmitted signal. A virtual line is drawn from each base station 302 along the respective arrival angle of the received signal. The position where the lines cross each other is estimated to be the position of mobile communication terminal 301.
FIG. 4 is a view showing the TDOA method, using the arrival time difference of a signal. A plurality of base stations 402, around a mobile communication terminal 401, receive a signal transmitted from the mobile communication terminal 401. Distances r41, r42, and r43 are calculated using the arrival time differences of the signal transmitted to base stations 402, and hyperbolic curves are drawn using the distances. The position where the hyperbolic curves cross each other is estimated to be the position of mobile communication terminal 401.
However, the position estimating method using the GPS requires a GPS receiver to be installed in the terminal, thereby increasing the production cost, size, and weight of the mobile communication terminal. In addition, the GPS receiver also uses the terminal's battery power, thereby increasing the power consumption of the terminal.
Also, with the position estimating method, the signal transmitted from the GPS satellite is very weak upon arrival at the terminal. Therefore, it is difficult to receive the GPS signal if there are obstacles between the terminal and the GPS satellite, such as in an urban area or indoor, whereby the functionality is lowered.
On the other hand, the network based position estimating method that does not modify the terminal does not require installation of the GPS receiver in the mobile communication terminal. The CDMA cellular network can be used as it is.
However, the TOA method, using the arrival time, and the TDOA method, using the arrival time difference, should use at least three base stations around the mobile communication terminal. The AOA method, using the arrival angle, should use at least two base stations. However, if the user is in a rural or suburban area, there is little chance that there are two or three base stations around the user.
Usually, there is only one base station in the cell where the user is located, and the other base stations are located far from the user. Therefore, the signal from the mobile communication terminal is difficult to receive by the other base stations and may be received with many errors. Therefore, it is difficult to estimate the position of the mobile communication terminal.
As described above, it is difficult to estimate the position of the mobile communication terminal using only one estimating method, such as the arrival time method, arrival angle method, or arrival time difference of signal method, in the rural or suburban area.